There is a growing need for air enriched with oxygen for industrial applications, in particular with a share between 25% and 35%. Combustion processes can be performed more efficiently in this manner. Membrane processes are suitable for oxygen enrichment. The membranes employed are semi-permeable, selective barriers which serve to separate gaseous or vaporous multicomponent mixtures. The substances preferably passing through the membrane are thereby enriched in the permeate and the substances held back by the membrane are located in the retentate. The substance separation takes place through the different permeation rates of the gases. A measure for the ability of a membrane to separate two gases is the ratio of their permeabilities, which is called selectivity. Particularly important for the cost effectiveness of a membrane technology is the use of a membrane with a high separation factor and a high gas flow, i.e., with a high selectivity and a high permeability. The separation factor is a material property, which can be increased for example through the development of special polymers, while the gas flow is a membrane property, which is among other things improved in that the effective thickness of a selective separation layer is reduced. However, a layer cannot be applied at just any thickness to a support membrane without provoking defects in this layer.
Moreover, in the case of long-term use of a membrane, the lowest possible fouling rate should be ensured, i.e., a low rate of deposition of dissolved substances on the outer membrane surface. Cost-effective production is also an important factor in the development of membranes.
Already known membranes are either not selective enough for an oxygen/nitrogen separation or they have a throughput that is too low, which requires membrane surfaces that are too large. A silicon composite membrane with a layer thickness of 1 μm has for example an oxygen flow of 1.6 m3/m2 h bar, but the oxygen/nitrogen selectivity is only 2.1. This is too low for most industrial applications. Membranes made of other polymers have a higher selectivity, but the oxygen flow generally lies far below 0.2 m3/m2 h bar, which requires membrane surfaces that are too large, see R. Baker: “Membrane Technology in the Chemical Industry: Future Directions”, Wiley-VCH, Weinheim, 2001, pages 268-295.
Furthermore, it was attempted to further develop membranes with a mixed matrix to the effect that zeolites, in particular silicalites, were introduced to cellulose acetate. The oxygen/nitrogen selectivity was thus increased from 3.0 to 3.6, see S. Kulprathipanja: “Mixed Matrix Membrane Development”, Annals of the New York Academy of Sciences, 2003, pages 361-369.
M. Jia et al. report in “Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation”, Journal of Membrane Science, 57, 1991, page 289-296, on membranes with a mixed matrix made of silicalite and PDMS (polydimethylsiloxane), which have a slightly increased oxygen/nitrogen selectivity.
Membranes with a mixed matrix, which are made of carbon molecular sieves and polyimides and which also have an improved combination of permeability and selectivity, are known from U.S. Pat. No. 6,562,110.
The aforementioned membranes are not suitable for use on an industrial scale. For example, one disadvantage is that the membrane properties could not be improved to the required degree through the introduction of additional filler material. Another disadvantage is that reproducible membrane production is not possible, since the filler material is not evenly distributed in the membrane. If the filler material particles also turn out to be too large, the membranes become too thick and no longer ensure sufficient permeability.